I have a simple server running Ubuntu Server. Usually the server connects to the network by Wifi, so there is usually no cable connected to the Ethernet port. I noticed that there is a dhclient3 running on eth0 anyway. It is causing undesirable disk activity. Obviously, it is never going to get a response as long as no cable is connected. I can kill the responsible process manually, but I wonder first what is starting the process and how I can prevent it from being started. If there is a way to get the server to boot without starting this process, I wonder whether this is the right solution. Connecting to the network by Ethernet is the failsafe when Wifi doesn't work. I guess what I would like to see would be a way for the server to detect when eth0 does not have a cable connected and not start dhclient in that case, but to activate dhclient when a cable is connected.

You might be interested in ifplugd: http://0pointer.de/lennart/projects/ifplugd/#overview
It monitors ethernet ports and automatically configs/deconfigs them when cables are connected. I have never used it, so I have no idea how (well) it works. If you try it, let us know how it goes!

Thanks. ifplugd looks like the solution. I also learned that these two lines in /etc/network/interfaces:

auto eth0
iface eth0 inet dhcp

are responsible for starting dhclient3 on eth0. Presumably, I am going to comment them out and substitute functionality from ifplugd.

Configuring ifplugd was fabulously easy.

1. apt-get install ifplugd

2. In /etc/default/ifplugd, add the name of the interface (e.g., eth0) to the INTERFACES= line.

3. In /etc/network/interfaces, comment out the corresponding auto line (e.g., auto eth0). I did not comment out the companion iface eth0 line.

I rebooted, but there is an inetd in /etc/init.d so I probably could have done a restart or force-reload on it.

Now, there is no dhclient.eth0.leases process running until I plug in the Ethernet cable. When I plug in the cable, the network connects in a few seconds.

Excellent - I'm glad to hear that it works as advertised, and solved your problem.

Cheers!